Typical prior art deflection systems in common use in television receivers include a fixed duty cycle horizontal oscillator coupled by a driver transistor and driver transformer to a horizontal output transistor. The horizontal output transistor is connected to a horizontal deflection winding contained in a yoke associated with a cathode ray tube. The objective of this circuitry is to deflect the electron beam or beams in the cathode ray tube in the horizontal direction at a relatively high rate such as 15,734 lines per second as is used in the NTSC system.
The design of the driver and horizontal output transistor stages is very critical. In typical circuits the horizontal output transistor conducts during the last half of the trace interval and is sharply cut-off to initiate retrace. During retrace a large retrace or flyback voltage appears at the collector of the horizontal output transistor. If the transistor is conducting when this voltage appears, the retrace current will be altered and the transistor will dissipate excessive power which generally will result in damage to the transistor. Since the horizontal output transistor is conducting heavily immediately before retrace is initiated, the stored charge in the base and collector regions of the transistor must be removed before the transistor will cease conducting collector current. Normally this stored charge is removed by applying a negative pulse to the base of an NPN transistor to cause a reverse base current to flow. This negative pulse, however, should be applied only as long as necessary to alleviate the voltage stress on the collector-base junction during retrace. R. J. Walker and R. Yu, "Horizontal Output Transistor Base Circuit Design," IEEE Transactions on Broadcast and Television Receivers, Vol. BTR-20, Aug. 1974, pp. 185-192, describe the various design requirements for the base circuit of the horizontal output transistor.
The above design requirements place severe limitations on the horizontal driver transistor and transformer design. In typical circuits conduction by the driver transistor during retrace and the first half of trace switches the output transistor off. The driver transformer stores energy during conduction by the driver transistor which is used to provide forward base current to the horizontal output transistor during the following conduction interval of the output transistor. Accordingly, the drive current in the primary winding must be of the proper amplitude and duration to maintain the horizontal output transistor in conduction in saturation during the second half of trace. When the driver transistor turns on again, the initial current flow through the primary winding of the driver transformer provides a turn-off pulse to the base of the output transistor. The amplitude and duration of the turn-off pulse are dependent on the inductances and the turns ratio of the driver transformer windings.
The critical design requirements of the driver and output stages also necessitates a well-regulated voltage supply for both the driver circuitry and the output circuitry. Accordingly, typical prior art horizontal deflection systems are energized by a voltage supply derived from rectification of the line voltage, but regulated so that the voltage levels vary within narrow limits. Typical prior art voltage regulators used for this purpose include series regulators and constant voltage power transformers as well as other similar circuitry.
One form of known voltage regulator is a switching regulator which regulates by varying the conduction time of a switch with the amplitude of the unregulated voltage. In horizontal deflection systems the horizontal output transistor is operated as a switch. Since the horizontal output transistor does not conduct during the entire trace interval, the portion that it does not conduct deflection current can be used for regulation. P. L. Wessel, "A New Horizontal Output Deflection Circuit," IEEE Transactions On Broadcast and Television Receivers, Vol. BTR-18, Aug. 1972, pp. 177-182, and M. J. Maytum, "Transistorized Self-Stabilizing Horizontal-Deflection Systems," IEEE Transactions On Broadcast and Television Receivers, Vol. BTR-20, Feb. 1974, pp. 32-64, describe various self-regulating horizontal deflection circuits. In such circuits the horizontal output transistor is isolated from the deflection winding during that portion of trace when the damper or trace diode is conducting. During this time, the output of the horizontal oscillator is pulse width modulated so that the conduction time of the horizontal output transistor varies over relatively broad limits sufficient to provide regulation of an unregulated voltage supply and regulation against load variations.
Self-regulating horizontal deflection systems, however, place even more severe requirements on the horizontal driver circuitry since the conduction and non-conduction intervals vary. When a low unregulated voltage requires a long conduction interval by the horizontal output transistor, the driver conducts for a shorter interval, yet must permit sufficient energy to be stored in the driver transformer to cause the output transistor to conduct in saturation for a longer interval. When the unregulated voltage is high, however, the driver conducts for a longer interval and causes excess energy to be stored in the driver transformer thereby providing excess base drive to the horizontal output transistor. Excess base drive, in turn, causes excess charge storage in the base and collector regions of the output transistor thereby varying the turn-off drive requirements. While some improvements in prior art drivers can be obtained by using current sources, the conflicting requirements of the turn-on and turn-off drives generally require deleterious compromises to be made. Furthermore, the base drive of the output transistor is generally excessive during most operating conditions thereby causing excessive power dissipation and heat generation.